Estimation of the diffuse component of daily global radiation in Valencia and Sevilla

The relationship between the diffuse fraction of daily global solar radiation and (1) the clearness index, (2) sunshine duration, and (3) the clearness index combined with fractional sunshine duration are obtained from the radiation data measured at Valencia and Sevilla, Spain. Two years of daily radiations data calculated from measured 20-s values are used to develop the relationship. The present correlations are in agreement with several correlations reported in the literature, developed for different locations. This good agreement indicates that the correlation is location independent. The present data confirm the seasonal dependence of the correlation. Three equations for a correlation between the daily diffuse transmission coefficient and the daily clearness index are presented.